Method for producing cell-fushion type morbillivirus mutants

ABSTRACT

It was found that a mutation of an amino acid at a specific position in the F protein of a  morbillivirus  induces a reduction in the cell-fusion ability. By introducing this mutation, a virus having a reduced cell-fusion ability can be produced. Thus, attenuated viruses useful in the preparation of vaccines can be easily produced.

TECHNICAL FIELD

The present invention relates to a method for reducing cell-fusion ability by a site-specific mutation of the morbillivirus-derived F protein. This F protein is useful in the attenuation of the virus.

BACKGROUND ART

The genus Morbillivirus is one of the genera under the family Paramyxoviridae of the order Mononegavirales, including many pathogenic viruses such as the measles virus that causes “measles”—an acute eruptive disorder. The measles virus widely infects infants, expressing symptoms such as fever, eruptions, cough, and such, occasionally causing severe complications such as measles-associated encephalitis, pneumonia, and such, sometimes even death. Furthermore, though very rarely, the measles virus sustains its infection even after the cure of infectious symptoms, causing encephalitis with a poor prognosis, named subacute sclerosing panencephalitis (SSPE). The one and only effective prophylactic means is vaccination with an attenuated measles virus vaccine.

The AIK-C strain, one of the attenuated measles virus vaccines, is a viral strain obtained by continual passage of the measles virus Edmonston strain in sheep kidney cells and chicken embryos cells. The AIK-C strain is excellent in its seroconversion rate as well as safety, which has earned it a high reputation internationally. With the spread of this attenuated measles vaccine, patients who contract measles have noticeably declined in number. In general, the seroconversion rate and safety of a vaccine are two incompatible characteristics, making it difficult to maintain both at a high standard. Therefore, if the mechanism of attenuation used in the AIK-C strain can be applied to other strains and viruses, it will be useful in the development of attenuated vaccines.

However, the mechanism by which this virus strain becomes attenuated remains unclear.

DISCLOSURE OF THE INVENTION

An objective of the present invention is to provide DNA to be used for producing a virus having a reduced cell-fusion ability. Another objective of this invention is to provide a method for reducing the cell-fusion ability of a virus by introducing a site-specific mutation into its F protein, and also to provide a virus having a reduced cell-fusion ability due to a site-specific mutation in its F protein. Viruses having a reduced cell-fusion ability produced by the method of this invention are useful in producing vaccines, and such, as attenuated viruses.

Since cell fusion is profoundly involved in the proliferation and propagation of viruses of the genus Morbillivirus, it is assumed that differences in this cell-fusion ability might affect the pathogenicity of the viruses. The present inventors thought that, if a certain mutation in a viral protein is causing a reduction in the cell-fusion ability, it may be possible to control viral proliferation and pathogenicity by identifying that mutation and producing a virus having such a mutant protein.

Therefore, the present inventors expressed the measles virus-derived F and H proteins with mutations introduced at several positions in cells to examine effects of those mutations on the viral cell-fusion ability. As a result, the inventors discovered that the 278^(th) amino acid in the F protein is closely associated with the cell-fusion ability. In the Edmonston strain that has a high cell-fusion ability, the 278^(th) amino acid in the F protein is phenylalanine. Furthermore, the inventors ascertained that, among F proteins having mutations at several positions, those having phenylalanine at the 278^(th) position similar to the Edmonston strain, express a cell-fusion ability equivalent to that of the F protein of the Edmonston strain, and that the cell-fusion ability can be significantly reduced by mutating this amino acid. Thus, this invention is the first to disclose a relationship between the 278^(th) amino acid in the F protein and viral cell-fusion ability.

Based on the above-described knowledge, the present inventors discovered that modification of an amino acid at the 278^(th) position in the viral F protein enables the reduction of viral cell-fusion ability. Viruses having a reduced cell-fusion ability are thought to become difficult to proliferate and propagate in hosts, which leads to viral attenuation. Isolation of attenuated viruses useful in the development of vaccines has hitherto relied on screening for mutant viral strains, which is a low-efficient and time-consuming procedure. The present invention makes it possible to easily attenuate any desired virus.

Namely, the present invention relates to DNA used in producing viruses having a reduced cell-fusion ability, a method for reducing viral cell-fusion ability by introducing a mutation into a specific amino acid in the viral F protein, and viruses having a reduced cell-fusion ability due to a mutation at a specific position in the F protein, and more specifically relates to each of the following inventions:

-   [1] a DNA according to (a) or (b), wherein the DNA is used to     produce a virus having a reduced cell-fusion ability:     -   (a) a DNA encoding a protein derived from the F protein of a         virus belonging to the genus Morbillivirus, wherein the protein         encoded by the DNA comprises an amino acid other than         plienylalanine at a position corresponding to the 278^(th)         position of a protein comprising the amino acid sequence set         forth in SEQ ID NO: 2, or     -   (b) a DNA encoding a protein having at least a 65% identity to         the amino acid sequence set forth in SEQ ID NO: 2, wherein the         protein encoded by the DNA comprises an amino acid other than         phenylalanine at a position corresponding to the 278^(th)         position of a protein comprising the amino acid sequence set         forth in SEQ ID NO: 2; -   [2] the DNA according to [1] that encodes a protein derived from the     measles virus; -   [3] the DNA according to [1], wherein the amino acid other than     phenylalanine is leucine; -   [4] a protein encoded by the DNA according to [1]; -   [5] a vector into which the DNA according to [1] is inserted; -   [6] the vector according to [5] used for reconstituting a measles     virus with a reduced cell-fusion ability; -   [7] a method for reducing the cell-fusion ability of a virus     according to (a) or (b), wherein the method comprises the step of     introducing a mutation to a position corresponding to the 278^(th)     amino acid in the protein comprising the amino acid sequence set     forth in SEQ ID NO: 2:     -   (a) a virus belonging to the genus Morbillivirus, or     -   (b) a virus comprising a F protein having at least 65% identity         to the amino acid sequence set forth in SEQ ID NO: 2; -   [8] the method according to [7], wherein the virus is the measles     virus; -   [9] the method according to [7], comprising substituting the amino     acid at a position corresponding to the 278^(th) position, with     leucine; -   [10] a virus with a reduced cell-fusion ability obtained by the     method according to [7]; -   [11] the virus according to [10], wherein the virus is an attenuated     virus; -   [12] a pharmaceutical composition comprising the virus according to     [11]; and, -   [13] the pharmaceutical composition according to [12] that is used     as a vaccine.

The present invention also relates to the use of DNA in a method for producing a virus having a reduced cell-fusion ability, in which the DNA encodes a protein derived from the F protein of a virus belonging to the genus Morbillivirus wherein the protein encoded by the DNA comprises an amino acid other than phenylalanine at a position corresponding to the 278^(th) position of a protein comprising the amino acid sequence set forth in SEQ ID NO: 2. This invention also relates to the use of the DNA in producing a virus with a reduced cell-fusion ability, in which the DNA encodes a protein having at least 65% identity to the amino acid sequence set forth in SEQ ID NO: 2 (Edmonston strain) and has an amino acid other than phenylalanine at a position corresponding to the 278^(th) position of a protein comprising the amino acid sequence set forth in SEQ ID NO: 2 (Edmonston strain). Furthermore, the present invention relates to the use of a vector having such a DNA in reconstituting a measles virus with a reduced cell-fusion ability.

The present invention relates to DNA used for producing a virus having a reduced cell fusion ability. The DNA of this invention include DNA encoding a protein derived from the F protein of virus belonging to the genus Morbillivirus and having an amino acid other than phenylalanine at a position corresponding to the 278^(th) position in the F protein of the measles virus Edmonston strain (SEQ ID NO: 2). The DNA of this invention also include DNA encoding a protein having at least 65% identity to the amino acid sequence of the F protein in the Edmonston strain (SEQ ID NO: 2) and having an amino acid other than phenylalanine at a position corresponding to the 278^(th) position in the F protein of the Edmonston strain. Identity to the amino acid sequence set forth in SEQ ID NO: 2 is preferably 80% or more, more preferably 95% or more. Amino acid sequence identity can be determined by the 3 Lipman-Person method using Genetyx-Mac Ver. 10 (Software Development).

Examples of viruses belonging to the genus Morbillivirus are the measles virus, canine distemper virus, phocid distemper virus, rinderpest virus, etc.

In this invention, a protein comprising an amino acid sequence having at least 65% identity to the amino acid sequence set forth in SEQ ID NO: 2 has a structure similar to that of a protein comprising the amino acid sequence described in SEQ ID NO: 2. Therefore, it can be assumed that phenylalanine at a position homologous to the 278^(th) position in SEQ ID NO: 2, similarly plays an important role in the maintenance of cell-fusion ability as the phenylalanine at the 278^(th) position in SEQ ID NO: 2. Furthermore, since viruses belonging to the genus Morbillivirus are taxonomically closely related to one another, the structure of the F proteins has been conserved among them. Therefore, phenylalanine at a position homologous to the 278^(th) position in SEQ ID NO: 2 can be assumed to play an important role in the maintenance of cell-fusion ability. Results of comparisons of amino acid sequences in the F proteins of viruses belonging to the genus Morbillivirus are shown in FIG. 4. Thus, an amino acid corresponding to the 278^(th) position in an amino acid sequence composing the F protein of each virus can be identified.

In this invention, a reduction in the cell-fusion ability of the F protein means a significant reduction in the ability of the F protein compared with an F protein having phenylalanine at a position homologous to the 278^(th) position. More specifically, the significant reduction is exemplified by a reduction of 50%, more preferably 70% or more. Herein, a reduction in the cell-fusion ability includes a complete loss of the ability. Cell-fusion ability of the F protein can be measured by, for example, a computer-assisted image analysis of virus-infected cell sizes under a microscope to calculate the ratio of areas occupied by cells having a predetermined size or more, and comparing these ratios expressed in numerals for each of virus strains. In the case of viruses such as the measles virus capable of growing in floating cells, cell sizes can be analyzed using a flow-cytometer, and such, to numerically express the results for comparison. Cell-fusion ability can be compared also by fixing cells with 0.5% glutaraldehyde, staining them with Giemsa, and observing the size of stained plaques under a microscope.

Cell-fusion ability is considered to be one of the important factors that determine viral proliferation and propagation abilities. Reduction in the cell-fusion ability of a virus is thought to cause a reduction in proliferation and propagation abilities of the virus, resulting in the achievement of viral attenuation. On the other hand, according to this invention, mutation of merely a single amino acid results in attenuation, and therefore, the structure as an antigen is maintained. Therefore, attenuation can be effectively achieved while maintaining the seroconversion rate at a high level.

The amino acid sequence of the F protein in the measles virus Edmonston strain and cDNA sequence encoding the protein are set forth in SEQ ID NOs: 2 and 1, respectively. In a protein of interest, a position homologous to the 278^(th) position in the F protein of the Edmonston strain can be determined by comparing the amino acid sequences. The position in a protein of interest need not be the 278^(th) position. For example, in the case of a protein having the structure of the F protein in the Edmonston strain that has been modified by, for example, an addition, insertion, and/or deletion of one or more amino acids, the homologous position may be a position other than the 278^(th) position. In such a protein, to determine a position homologous to the 278^(th) position in the F protein of the Edmonston strain, amino acid sequences of both proteins are aligned so as to match mutual amino acids as well as amino acids having similar properties as much as possible by inserting appropriate gaps in both amino acid sequences if necessary. Thus, it can be determined which position in a protein of interest corresponds to a position homologous to the 278^(th) position in the F protein of the Edmonston strain. Such a technique has been known among those skilled in the art, and can be performed easily using commercially available or published computer software, for example, the analytical software GENETYX-MAC VER. 10 (Software), etc.

DNA encoding a protein of interest comprising an amino acid other than phenylalanine at a position corresponding to the 278^(th) position in the F protein of the Edmonston strain is used for producing viruses having a reduced cell-fusion ability according to this invention. There is no particular limitation on the origin of these DNA, which may be naturally occurring DNA or DNA into which a mutation has been artificially or spontaneously introduced. Alternatively, they may be DNA comprising artificially designed sequences.

The DNA of the present invention can be prepared using, for example, hybridization techniques well-known in the field (Sambrook, J., Fritsch, E. F., and Maniatis, T. Molecular Cloning: A Laboratory Manual (2^(nd) edition). Cold Spring Harbor Laboratory Press, Cold Spring Harbor). DNA can also be isolated using the polymerase chain reaction technique (Sambrook, J., Fritsch, E. F., and Maniatis, T. Molecular Cloning: A Laboratory Manual (2^(nd) edition). Cold Spring Harbor Laboratory Press, Cold Spring Harbor).

That is, those skilled in the art can isolate DNA by screening virus-derived DNA, and such, using the hybridization technique and PCR method. Nucleotide sequences of probes necessary in the hybridization method and primers required in the PCR method can be designed based on, for example, the cDNA sequence (SEQ ID NO: 1) of the F protein of the Edmonston strain. By identifying the position in the amino acid sequence encoded by the isolated DNA, which is homologous to the 278^(th) position in the F protein of the Edmonston strain, a DNA encoding a protein having an amino acid other than phenylalanine at that position can be readily prepared.

By appropriately modifying the DNA thus obtained, the amino acid in the protein encoded by the DNA, which is at a position corresponding to the 278^(th) position in the F protein of the Edmonston strain, can be substituted with any desired amino acid other than phenylalanine. Alternatively, the introduction of a mutation so as to delete the phenylalanine is also included in this invention. The amino acid used for the substitution, can be appropriately selected. As described in Examples, a protein having an amino acid that was substituted with leucine at a position homologous to the 278^(th) position in the F protein of the Edmonston strain expressed a significant reduction in its the cell-fusion ability. Therefore, DNA encoding a protein having an amino acid that was modified with leucine at a position homologous to the 278^(th) position in the F protein of the Edmonston strain can be preferably used in the present invention.

Furthermore, the present invention proved that an amino acid at this position in the F protein has an important function in the cell-fusion ability. Therefore, in the case where an amino acid other than phenylalanine is present at the corresponding position, the cell-fusion ability may be further reduced or, reversely, the degree of the reduction of cell-fusion ability may be lowered by further mutating this amino acid to another amino acid. In this regard, a reduction in the cell-fusion ability includes the loss of the ability as well.

Methods for introducing a mutation into an amino acid in a protein are well known. For example, DNA encoding a desired amino acid sequence can be isolated by preparing a viral library comprising mutant viruses, DNA library encoding mutant F proteins, and such, and screening them for the desired DNA. Alternatively, mutant viruses can be screened from nature. Furthermore, site-specific mutagenesis can be performed using well-known genetic engineering techniques. For the introduction of site-specific mutations, for example, the SOE (splicing-by-overlap-extension)-PCR method (Ho, S. N., Hunt, H. D., Horton, R. M., Pullen, J. K., and Pease, L. R. (1989) Gene 77, 51-59), and Kunkel method (Kunkel, T. A. (1985) Proc. Natl. Acad. Sci. U.S.A. 82 (2): 488-92) can be used.

In addition, in the present invention, as long as the amino acid at a position corresponding to the 278^(th) position in the F protein is any amino acid other than phenylalanine, a position other than that may be further modified. For example, the DNA of this invention include DNA encoding proteins having an amino acid other than phenylalanine at the 278^(th) position of the F protein in the Edmonston strain (SEQ ID NO: 2) and also having one or more substitution, deletion, insertion, and/or addition of amino acids other than that at the 278^(th) position. In the case of the artificial modification of amino acids in the F protein encoded by DNA obtained from viruses belonging to Morbillivirus, the number of amino acids modified is usually five or less, preferably three or less, even more preferably, one amino acid excluding the one at the position corresponding to the 278^(th) position. Such an amino acid modification can be performed, for example, aiming at further reducing the cell-fusion ability of the F protein, and also aiming at improving the manipulability of DNA, for example, by the insertion of a restriction enzyme site, and such, and also with the aim of modifying a property of the F protein other than its cell-fusion ability. Mutations of amino acids in proteins may occur also in nature.

In general, to minimize the loss of properties of a protein as much as possible, an amino acid used for substitution is thought to be preferably one with a property similar to the substituted amino acid. For example, Ala, Val, Leu, Ile, Pro, Met, Phe, and Trp are all classified into the non-polar amino acid group, and thought to have similar properties. Furthermore, non-charged amino acids are exemplified by Gly, Ser, Thr, Cys, Tyr, Asn, and Gln. Acidic amino acids are exemplified by Asp and Glu, and basic amino acids by Lys, Arg, and His.

Furthermore, the present invention relates to proteins encoded by the DNA of this invention. Viruses with a lowered cell-fusion ability can be produced using the proteins of this invention. A protein of this invention can be expressed by inserting DNA encoding the protein into an appropriate expression vector, and introducing the vector into host cells. In the measles virus, and such, viruses with a reduced cell-fusion ability can be reconstituted from vectors having DNA encoding the proteins of this invention. Several methods for reconstituting the measles virus from cDNA have been reported, namely, the method of Radecke (Radecke, F., Spielhofer, P., Schneider, H., Kaelin, K., Huber, M., Dotsch, C., Christiansen, G. and Billeter, M. A. (1995) EMBO J. 14(23): 5773-84) and the method of Schneider (Schneider, H., Spielhofer, P., Kaelin, K., Dotsch, C., Radecke, F., Sutter, G. and Billeter, M. A. (1997) J. Virol. Methods 64(1): 57-64). According to these methods, a measles virus can be reconstituted from DNA encoding the N, P, M, F, H, and L proteins of the measles virus. Therefore, by using the DNA of this invention as DNA encoding this F protein, a measles virus with a reduced cell-fusion ability can be reconstituted. That is, transcription of DNA encoding the N, P, M, F, H, and Lproteins allows their transcription products to function as genomic RNA of the measles virus, so that measles viral particles can be formed in the presence of the N, P, and L proteins. The virus thus obtained can be further amplified by infecting the virus to appropriate hosts.

More specifically, RNA is extracted from a measles virus strain to synthesize cDNA. Using the known method of introducing a site-specific mutation, cDNA having a mutation (for example, a substitution with leucine) introduced into a position corresponding to the 278^(th) amino acid of the F protein of the wild type strain is prepared. From this cDNA, together with other cDNA coding for other measles virus genes, genomic cDNA corresponding to the entire viral genomic RNA is constructed using the restriction enzyme sites, and cloned downstream of the T7 promoter. Host cells infected with the T7 RNA polymerase-expressing vaccinia virus are cotransfected with a plamid having the above-described viral genomic cDNA and other plasmids into which the N, P, and L genes essential for the transcription and replication of the measles virus are cloned downstream of the T7 promoter, and cultured for a few days. From the culture supernatant thereof, a recombinant measles virus with a reduced cell-fusion ability is recovered. This method enables the attenuation of any measles virus strain. Viruses thus produced can be further amplified in appropriate hosts.

Several methods for reconstituting morbilliviruses other than the measles virus are known. For example, the method of Baron, et al. (Baron, M. D., and Barrett, T. (1997) J. Virol. 71(2): 1265-71); the method of Kai, et al. (Kai, C., Miura, R., Shimizu, F., Sato, H., Fujita, K., Hatama, S., Ohashi, K., Kamima, T., and Takahashi, E. Abstracts of the 47^(th) General Assembly of the Japanese Society for Virology (1999), p.289: Preparation of recombinant canine distemper virus using the reverse genetic method), and furthermore, Patent WO97/16538 are known.

In addition to the present invention, it is possible to introduce thermosensitivity into a virus by mutating the viral P protein together with the reduction of cell-fusion ability. The present inventors have proved that a thermosensitive characteristic can be introduced into morbilliviruses using a protein having an amino acid other than leucine at a position corresponding to the 139^(th) position of the P protein of the measles virus Edmonston strain. Combination of this knowledge with the instant invention enables one to alter the thermosensitive characteristic together with the reduction in the cell-fusion ability, providing extremely safer vaccine preparations.

In addition, the present invention relates to a method for reducing viral cell-fusion ability. The method of this invention is characterized by introducing a mutation to the amino acid at a position homologous to the 278^(th) position in the F protein (SEQ ID NO: 2) of the measles virus Edmonston strain, either in the F protein of viruses belonging to the genus Morbillivirus, or a protein having at least 65% identity to the amino acid sequence of the F protein in the Edmonston strain. Cell-fusion ability of viruses obtained by the present invention can be compared by, for example, performing a computer-assisted image analysis of sizes of virus-infected cells observed under a microscope, calculating the ratio of areas occupied by cells having a predetermined size or more, and numerically expressing the results for each viral strain. Furthermore, in the case of viruses such as the measles virus capable of growing in floating cells, the cell-fusion ability can be compared by analyzing the cell size using a flow cytometer and such, and numerically expressing the result. The comparison can be also performed by fixing cells with 0.5% glutaraldehyde followed by the Giemsa staining, and observing sizes of the stained plaques under a microscope.

Viruses thus obtained having a reduced cell-fusion ability are less pathogenic because their proliferation and propagation abilities in hosts are reduced. These viruses are extremely useful for producing safe live vaccines. According to the present invention, any virus strain can be attenuated by modifying its F protein using genetic engineering technology. When a virus of this invention is used as a pharmaceutical composition such as a vaccine, besides the use of the virus itself as a drug, it can be formulated by applying a known pharmaceutical procedure. For example, the virus may be administered as a pharmaceutical preparation by appropriately combining with a pharmacologically acceptable carrier or media, more specifically, sterilized water, physiological saline, a plant oil, emulsifier, suspending agent, surfactant, stabilizer, etc. When using the virus as a vaccine, it can be administered suitably in combination with an adjuvant. Administration to patients can be performed by methods known to those skilled in the art, for example, besides the intra-arterial, intravenous, and subcutaneous injections, it can be given intranasally, transbronchially, intramuscularly, or orally. Doses may vary depending on the weight and age of patients as well as the method of administration, purpose of usage, and so on, and may be appropriately selected by one skilled in the art.

In general, in Japan, the vaccine strain of the measles virus is cultured by inoculation to cultured chicken embryos cells prepared from embryonated eggs produced in SPF facilities approved by the Japanese Ministry of Health and Welfare. After the culture, a stabilizer is added to a vaccine solution that has cleared the germ-free test, and purified to obtain an undiluted vaccine concentrate. This vaccine concentrate is stored at −80° C., and at the same time examined for its safety and efficacy. Vaccine concentrates that have cleared the test are pooled as the final bulk, from which vaccine preparations are made. Those that have cleared repeated national tests and private tests are sold as the final preparation.

Furthernore, the viruses of the present invention can be used as vectors for gene therapy.

All the prior art literatures cited herein are incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents schematic diagrams showing the results of measuring cell-fusion ability F protein mutants. Numerals and alphabets described represent the position of mutations in the amino acid sequences of the F proteins deduced from the nucleotide sequences of pAIK-F01 and pEdm-F, and the amino acids at the positions, respectively. A cell-fusion ability similar to that expressed by the F region plasmid pEdm-F derived from the Edmonston strain is shown with +++.

FIG. 2 represents photographs showing the results of cell-fusion observations with F proteins derived from the AIK-C and Edmonston strains, respectively. The photograph marked “278 Leu” shows the result of transformation with a plasmid comprising cDNA encoding a protein having leucine at the 278^(th) position thereof. “B95a Mock” shows the result in the case of infection only with the vaccinia virus (vTf7-3).

FIG. 3 represents photographs showing the results of plaque assays using Vero cells, in which the infectious viral particles were recovered from a cDNA clone in which a mutation was introduced into the F protein, which was obtained from an infectious clone constructed from the AIK-C strain. Herein, the upper panels show the assay results at the culture temperature 39° C., while the lower panels represent the results at the culture temperature 33° C.

FIG. 4 represents the results of comparing amino acid sequences of viral F proteins belonging to the genus Morbillivirus. Positions homologous to the 278^(th) position are enclosed in the box (SEQ ID NOs. 5-9). Shown from the top are amino acid sequences of F proteins of the AIK-C strain (SEQ ID NO. 10), Edmonston strain (SEQ ID NO. 11), canine distemper virus (SEQ ID NO.12), phocid distemper virus (SEQ ID NO. 13), and rinderpest virus (SEQ ID NO. 14).

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in more detail below with reference to examples, but it is not to be construed as being limited thereto.

EXAMPLE 1 Comparative Analysis of Cell-Fusion Ability of F Protein

From the AIK-C strain passaged in Vero cells and its parent Edmonston strain, the F and H coding regions were amplified by PCR respectively, and were inserted downstream of the T7 promoter of the plasmid BSSK (Short, J. M., Fernandez, J. M., Sorge, J. A. and Huse, W. D. (1988) Nucleic Acids Res. 16 (15), 7583-7600) to construct the F protein-expression plasmids pAIK-F01 and pEdm-F as well as H protein-expression plasmids pAIK-H and pEdm-H. The amino acid sequence of the F protein of the Edmonston strain and cDNA sequence encoding the protein are set forth in SEQ ID NOs: 2 and 1, respectively, and the amino acid sequence of the F protein of the AIK-C strain and cDNA sequence encoding the protein are set forth in SEQ ID NOs: 4 and 3, respectively. After the infection of B95a cells with the vaccinia virus expressing the T7 RNA polymerase, the cells were cotransfected with these expression plasmids, and cell fusion abilities were examined.

TABLE 1 F protein-expression H protein-expression plasmid plasmid Cell fusion pAIK-F01 pAIK-H + PAIK-F01 pEdm-H + pEdm-F pAIK-H +++ pEdm-F pEdm-H +++ +:Cell fusion 25% or less ++:Cell fusion 25-50% +++:Cell fusion 50% or more

As shown in Table 1, as a result of examining various conbinations of plasmids expressing the F protein and H protein, it became obvious that the magnitude of cell-fusion ability is controlled by the F protein.

To further confirm effects of amino acids at the 278^(th) position in the F protein on cell-fusion, the experiment was performed with the AIK-C strain using the plasmids pAIK-F01 and pEdm-F. As described above, in the pAIK-F01 thus constructed, the amino acid at the 362^(nd) position was replaced with Tyr (Y) compared to the amino acid sequence (SEQ ID NO: 4) deduced from the nucleotide sequence (SEQ ID NO: 3) of the AIK-C strain previously reported by Mori et al. In the pEdm-F constructed from the Edmonston strain, differences in amino acids were detected at six positions compared to pAIK-F01. These two plasmids were recombined using the restriction enzyme Bst1107 to construct pMV-F-SVS and pMV-F-VFY. Both plasmids were further recombined using Bst1107 and PshAI to construct pMV-F-VF. This plasmid and pAIK-F01 were recombined with SalI to construct pMV-F278Phe.

Using these plasmids and the restriction enzymes SalI, Bst1107, and PshAI, pMV-F-VL was constructed. Using pAIK-H as the H protein expression partner and each of the plasmids, cotransfection was done to determine the cell-fusion ability. As a result, pEdm-F, pMV-F-VFY, pMV-F-VF, pMV-F-278F, which have Phe as the amino acid at the position 278, showed a high cell-fusion ability(FIG. 1). From the aforementioned facts, it became apparent that the low cell-fusion ability of the AIK-C strain is controlled by the amino acid at the 278^(th) position in the F protein. That is, the amino acid at the 278^(th) position in the F protein of the wild type strain which is Phe (F) results in a high cell-fusion ability. On the other hand, in the AIK-C vaccine strain, this position is mutated to Leu (L), and therefore, this mutation was thought to bring about a reduction in the cell-fusion ability, becoming one of the causes for attenuation.

Cell fusion patterns of cells expressing pAIK-F01 encoding Leu at the 278^(th) position, pMV-F278Phe encoding the F protein of the AIK-C strain in which the amino acid at the 278^(th) position is mutated to Phe (marked with pMV-F278F in the figure), and pEdm-F encoding the F protein of the Edmonston strain are shown in FIG. 2. When Phe is at the 278^(th) position, a formation of remarkable syncytia due to cell-fusion was observed, while, when Leu is at the 278^(th) position (marked with pAIK-F01 in the figure), significantly small syncytia were observed.

EXAMPLE 2 Preparation of Measles Virus with Lowered Cell-Fusion Ability

An infectious cDNA clone system was developed based on AIK-C strain and infectious recombinant viruses were recovered from a cDNA genome into which an F protein mutation was introduced. Then, the plaque assay was carried out by using vero cells.

A full-length genome cDNA was constructed based on the measles vaccine AIK-C genome, named MVAK-F278Leu (carrying the F gene of pAIK-F01 in Example 1) in which the amino acid at the 278^(th) position in the F protein region is Leu same as that in the AIK-C strain. Then, a full-length genome cDNA named MVAK-F278Phe (carrying the F gene of pMV-F278Phe in Example 1) having the amino acid at the 278^(th) position of the F protein replaced with Phe was constructed. These genome cDNAs were cotransfected to B95a cells together with the N-, P-, and L-expression plasmids, and the supernatant was cultured over two generations to recover the infectious recombinant viruses. Viruses thus recovered were infected to Vero cells and cultured at 37° C. for the plaque assay.

Results are shown in FIG. 3. At 33° C., the recombinant virus into which F278 Phe had been introduced formed large plaques (MVAK-F278Phe), while the virus having the same F278 Leu as the vaccine strain expressed small plaques (MVAK-F278Leu). Most of the plaques formed by the AIK-C vaccine virus at 33° C. were small (AIK-C, 33° C.) with a few large-sized plaques. Furthermore, no plaque was observed with these viruses at 39° C. On the other hand, the parent Edmonston strain expressed large-sized plaques at both 33° C. (Edm, 33° C.) and 39° C. (Edm, 39° C.).

INDUSTRIAL APPLICABILITY

The present invention provides DNA used for producing a virus with a reduced cell-fusion ability, a method for reducing the viral cell-fusion ability by a site-specific mutagenesis of the viral F protein, and viruses with a reduced cell-fusion ability prepared by a site-specific mutation in the F protein. Thus, the instant invention enables the easy production of attenuated viruses, thereby enables the speedy development of live vaccines against novel newly emerging viruses.

Besides the measles virus, the genus Morbillivirus in particular includes many pathogenic viruses, such as the canine distemper virus, phocid distemper virus, and rinderpest virus. Therefore, the method of the present invention capable of attenuating viruses of the genus Morbillivirus with a minimal mutation is highly useful in the development of vaccines. 

1. A method for reducing the cell-fusion ability of a virus comprising introducing a mutation in vitro to a position corresponding to the 278^(th) amino acid in a protein of the amino acid sequence set forth in SEQ ID NO: 2; wherein the virus is of the genus Morbillivirus; or the virus comprises an F protein having at least 65% identity to the amino acid sequence set forth in SEQ ID NO: 2; wherein the method does not include converting the virus to a virus identical to measles virus AIK-C.
 2. The method according to claim 1, wherein the virus is a measles virus.
 3. The method according to claim 1, comprising substituting the amino acid at a position corresponding to the 278^(th) position with leucine.
 4. The virus with a reduced cell-fusion ability obtained by the method according to claim
 1. 5. The virus according to claim 4, wherein the virus is an attenuated virus.
 6. A composition comprising the virus according to claim 4, and pharmacologically acceptable carrier or media.
 7. The method of claim 1, wherein the virus is a distemper virus or rinderpest virus.
 8. The method of claim 1, wherein the protein encoded by the gene is different than the F protein of measles virus AIK-C strain.
 9. The method of claim 1, further comprising five or less additional substitutions in the protein.
 10. The method of claim 1, further comprising three or less additional substitutions in the protein.
 11. The method of claim 1, further comprising one additional substitution in the protein.
 12. The method of claim 1, wherein the protein is changed only at the position corresponding to the 278^(th) position.
 13. The method of claim 1, wherein only the F gene is changed.
 14. The method of claim 1, wherein only the F and the P genes are changed.
 15. A method of producing a DNA encoding mutant protein having a reduced cell fusion ability, comprising introducing a mutation in vitro so that the mutant protein encoded by the DNA comprises an amino acid other than phenylalanine at the position corresponding to the 278^(th) position of a protein of the amino acid sequence of SEQ ID NO: 2; wherein the protein is a protein derived from the F protein of virus belonging to the genus Morbillivirus, wherein the protein has phenylalanine at the position corresponding to the 278^(th) position of a protein comprising the amino acid sequence of SEQ ID NO: 2; or an F protein having at least 65% identity to the amino acid sequence set forth in SEQ ID NO: 2, wherein the protein has phenylalanine at the position corresponding to the 278^(th) position of a protein comprising the amino acid sequence of SEQ ID NO:
 2. 16. The method of claim 15, wherein the protein encoded by the DNA is not converted to a protein identical to the F protein of measles virus AIK-C strain.
 17. The method of claim 15, further comprising five or less additional substitutions in the protein.
 18. The method of claim 15, further comprising three or less additional substitutions in the protein.
 19. The method of claim 15, further comprising one additional substitution in the protein.
 20. The method of claim 15, wherein the protein is changed only at the position corresponding to the 278^(th) position.
 21. The method of claim 15, wherein the protein is derived from a measles virus.
 22. The method of claim 15, wherein the protein is derived from a distember virus or a rinderpest virus.
 23. The method of claim 15, wherein the amino acid at the position corresponding to the 278^(th) position is substituted with leucine.
 24. An isolated DNA produced by the method of claim 15 or a replicated DNA thereof, wherein the DNA encodes a protein other than the F protein of measles virus AIK-C strain.
 25. An isolated protein encoded by the DNA of claim
 24. 26. A vector having the DNA of claim
 24. 27. The vector of claim 26, wherein the vector encodes a viral genomic RNA.
 28. A proliferated virus of the virus according to claim 4, having the mutation at the 278^(th) amino acid in the F protein, and with a reduced cell-fusion ability. 